Abstract: A digital power amplifier has a digital signal processor (2) providing a digital signal representative of the original input (I). A pulse width modulation counter (5) applies a pulse width modulated output waveform to the output switching stage (6) which applies an output to a load (L) powered by a power source (7). The power source (7) is coupled to a frequency modulator (8) whose output is arranged to clock the counter (5). In this way, noise components from the power supply (7) are modulated onto the input signal applied to the output switching stage (6) and act to cancel out the noise signals applied to the output stage by the power supply, whereby an output (O) generally free of noise attributable to the power source is produced.